Routing Protocol Tips!

patelrasesh Network Architecture, Network Design, Routing, , , , , , , ,
  • 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.

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