Routing Timers

EIGRP

• It is possible for two routers to become EIGRP neighbors even though the hello and hold timers do not match. The hold time is included in the hello packets so each neighbor should stay alive even though the hello interval and hold timers do not match.
• If you change the hello interval, the hold time is not automatically adjusted to account for this change – you must manually adjust the hold time to reflect the configured hello interval.
• The hold time is typically three times the hello interval.

**You can configure as low as 1 second hello and 3 second hold interval.

OSPF

• OSPF requires these intervals to be exactly the same between two neighbors. If any of these intervals are different, these routers will not become neighbors on a particular segment.
• The dead interval is typically four times the value of the hello interval.

**OSPF fast hello packets are achieved by using the ip ospf dead-interval command. The dead interval is set to 1 second, and the hello-multiplier value is set to the number of hello packets you want sent during that 1 second, thus providing subsecond or “fast” hello packets.

IS-IS

• Hello intervals and hold times donot have to match between IS-IS neighbors for an adjacency to form.
• By default, an IS-IS router waits 3 times the hello interval until it considers a neighbor dead. (The isis hello-interval and the isis hello-multiplier commands are changed on a per-interface basis. Timers can vary on different interfaces.)

** A faster hello interval facilitates faster convergence but increases bandwidth and CPU use. It might also add to instability in the network. A slower hello interval saves bandwidth and CPU use.

BGP

• You can configure two BGP neighbors with different hold timers. The lowest value out of two is accepted.
• After the TCP connection is up, the routers send open messages in order to exchange values. The values that the routers exchange include the AS number, the BGP version that the routers run, the BGP router ID, and the keepalive hold time. After the confirmation and acceptance of these values, establishment of the neighbor connection occurs.
• The hold time is typically three times the keepalive interval.

**You can configure as low as 20 second keepalive and 60 second hold timer.

**Hardware base BFD is best option to use instead of routing timers for fast convergence.

BFD is a detection protocol designed to provide fast forwarding path failure detection times for all media types, encapsulations, topologies, and routing protocols. In addition to fast forwarding path failure detection, BFD provides a consistent failure detection method for network administrators.

Routing Protocol Tips!

• If using multiple points for redistribution between different IGP protocols makes sure to use filters or route tag.
• BFD is the best solution for fast failure detection for the neighbours (routing) which are not using point-to-point links.
• For optimal routing, reduce the routing table / database by summarization or filtering at correct place.
• Always keep in mind to double check for unknowingly created routing black holes after route summarization, has summarization creates null routes automatically. (For OSPF you can disable creation of null routes by command “no discard-route”).
• Layer 3 Designs document should include points or remarks where summarization, filtering or redistribution is done along with the router type (e.g. Stub, ABR, ASBR etc.).

EIGRP

• Best suited for Hub and Spoke Design.
• Reduce query domains by configuring route summarization or filtering.
• Configure spoke / remote edge routes has stub router if possible (for dual home sites you can use leak-map to configure them has stub).
• Try not to have more than 3 or 4 feasible successor.

**EIGRP router reflect (Over the Top)

OSPF

• Make sure Area 0 is contiguous if not use virtual link’s (if no other alternative then only use virtual links to interconnect area 0).
• Try to configure stub area where ever design allows.
• Try to use partial SPF @ Edge / Leaf Nodes.
• Incremental SPF, SPF Run Time, Event Propagation Time, LSA timers (throttle lsa all, arrival etc.).
• Link State Loop Free Alternatives.
• Use OSPF priority instead of router ID for DR election process.

IS-IS

• If planning to have single IS-IS domain design then go for L2 domain (L2 give flexibility for future redesign).
• L2 domain need’s to contiguous (same has OSPF area 0).
• L1 domain contains intra intra area routes, default route advertised by L2 domain and redistributed routes (similar to OSPF NSSA).
• If you are redistributing IS-IS into any routing protocol and if you have connected interface advertised only to IS-IS it will not redistribute connected interface route.
• If you are running MPLS make sure to use metric-style wide. If you are in a transition process use metric-style transition.
• IS-IS prefix prioritization is one of the tools you can use for fast convergence.
• Consider IS-IS mesh-group option for a fully mesh network running IS-IS
• IS-IS DIS is similar to OSPF DR (Diff: IS-IS don’t have backup DIS while OSPF has backup DR router)

BGP

• Reason to use BGP should be to break complexity into smaller pieces not because of excessively huge routing table / database.
• BGP will be able to handle the huge amount of routes but it will not address underline issue if selection of BGP was done to just overcome the size of the routing table.
• Routes learned from an eBGP peer propagated to all peers.
• Routes learned from iBGP peer propagated only to eBGP peers.
• Locally generated routes propagated to all peers.
• iBGP cannot detect routing loops within an AS.
• For smaller iBGP networks with limited growth, iBGP full mesh is easy and cleaner solution.
• If using loopback address to form eBGP neighbours don’t forget ebgp-multihop.
• Use route dampening and peer group has per your requirements. (peer group help if sending same advertisement to all peer’s)
• Load Sharing for eBGP routes
  • Outbound
    • Two Static routes for same subnet or default pointing to different neighbour (next hop IP)
    • Best route selection on base of BGP attributes for each prefix.
    • Accept partial table, to accept only those routes from each provider that are directly attached to them.
  • In-bound
    • AS path prepend
    • Set MED outbound
    • Set communities (will need to get ISP involved to route traffic has per community value)

**Last two only apply if you are dual-homed to the same provider.

• Alternate to iBGP full mesh
  • Confederations
    • Recommended to deploy when you building a brand new network
    • Will required extended amount of outage window if configured / deployed on existing network running IGP or iBGP mesh.
    • Sub-AS are treated as eBGP neighbours (local pref, MED & next hop is unchanged between sub-AS).
  • Route Reflectors
    • Place router reflectors in flow of physical topology.
    • You can place RR out of flow of physical topology in certain exceptions such has when packet forwarding is achieved through label switching or IP tunneling
    • Easy to deploy with minimum outage window if configured / deployed on existing network running IGP or iBGP mesh.
    • Originator id & Cluster list are used for loop prevention.
    • Cannot modify path attributes for iBGP learned routes when exchanging routing information to other iBGP peers / clients.

What is Confederations?

Confederation is one of the method to reduce number of iBGP neighbors (or get away from full iBGP mesh)

• Confederations make the network look like one AS to the eBGP peers.
• The AS is divided into pieces, and each piece is its own AS (private AS)
• The iBGP peers are the other BGP speakers in the same sub-AS, whereas the eBGP peers are the BGP speakers in the other sub-autonomous systems and outside the main AS.

Advantages

• Better control of the policies within the network. (A sub AS can be fully mesh or can even have a route reflector)
• Easy to deploy / configure when build a network from scratch.

Note:

• Improper or misconfigured confederation can cause routing loops.
• At all times, each member of a confederations should know what the real ASN is, which sub-autonomous system it belongs to, and what other sub-autonomous systems belong to the same confederation.
• Local preference and MED are pass along with NEXT_HOP is unchanged between sub-autonomous systems.

What is Router Reflector?

Rule: “iBGP peers cannot advertise routes learned from iBGP peers to other iBGP peers”, does not apply to router reflector.

• A router is configured as a router reflector, and the other iBGP routers, known as clients, peer with the router reflector only. A router reflector and its clients are known collectively as a cluster.
• Router reflector can peer with both iBGP (client, non-client) and eBGP outside the cluster.
• Route reflector offer an alternative to fully meshed iBGP peers.

Route Reflections Rules

• If the route was learned from a non-client iBGP peer, it is reflected to clients only.
• If the route learned from a client or eBGP peer, it is reflected to all non-clients and clients.

Route Reflector Loop Prevention

1. ORIGINATOR_ID (it will carry the ROUTER_ID of the originator of the route in the local AS)
2. CLUSTER_LIST (when a RR reflects a route from its clients to a non-client peer, it must append the local CLUSTER_ID to the CLUSTER_LIST)

Note:

• To avoid possible routing loops or other routing errors, the router reflector could not change the BGP path attributes.
• When exchanging internal routing information between RRs and clients and non-clients. RR can modify BGP path attribute, NEXT_HOP for eBGP learned routes send to its internal peers.
• A rule of thumb is to place RR in flow of physical topology. In other words, define the iBGP peering to match the physical / topology connectivity of the network. (in-path)
• You can place RR out of flow of physical topology in certain exceptions such has when packet forwarding is achieved through label switching or IP tunneling
• 

What is Route Server?

Some times route reflectors are confused with route servers (and vice versa). Route servers are generally used at Internet exchange points. the objective is for routers to peer only with the route server and obtain all the routing information from it. the route server has the capability of propagating information in a transparent fashion, as if the advertisements were received directly from the router originating it.

Route reflectors also try to reduce the number of peers needed in an iBGP cloud, where as the the route server is typically used with eBGP neighbors.

The route server process no traffic.

http://routeserver.org/

What is BGP?

Border Gateway Protocol (BGP) = Path Vector / Distance Vector Routing Protocol

• BGP is mostly used by Internet service providers.
• eBGP = BGP used to route between different autonomous systems (AS).
• iBGP = BGP used to route within an autonomous systems (AS).

Route Advertisement

• Routes learned from an eBGP peer are propagated to all peers.
• Routes learned from an iBGP peer are propoagated only to eBGP peers.
• Routes originated locally are propagated to all peers.

BGP Path Selection Preference

1. Weight
2. Local_Pref
3. Origination
4. AS_PATH
5. ORIGIN
6. MED
7. eBGP
8. iBGP
9. Lowest IGP metric to the BGP next-hop
10. Maximum-paths prefer first path lowest router-id
11. Minimum cluster_list lowest neighbour address

Note:
BGP cannot detect routing loops within an AS, it can detect loops only in eBGP routes. Because of this you should not redistribute iBGP routes into your IGP

Default Routing Administrative Distance