Monday, July 26, 2010

BSCI - Route Summarisation with RIPv2, EIGRP, OSPF, IS-IS, BGP

Something I seem to be struggling with is committing to memory how to implement route summarisation within each routing protocol. So in this post I'm going set down the necessary steps to implement summarisation in each protocol.

On the router running RIPv2 you apply the summary to the outgoing interface:
R1(config)#int s0/0
R1(config-if)#ip summary-address rip

Firstly disable auto summarisation-
R1(config)#router eigrp 1
R1(config-router)#no auto-summary

Next apply your summary route to the OUT Bound interface -
R1(config)#int s0/0/0
R1(config-if)#ip summary-address eigrp 1 255.255.224.

Points - Configured on a per interface basis, Router creates a NULL0 entry as loop prevention (longest match rule works here so if there isn't a more specific route within the summary route the router will drop the packet), When the last specific route within the summary is removed the NULL0 entry is removed.

Summarisation is performed on the ABR and ASBR only.
ABR - Applied to routes from within one area and summarised in to another area
ASBR - External routes are redistributed INTO ospf from protocols such as RIPv2 or EIGRP

For Intra-area route summarisation, on the ABR do:
R1(config)#router ospf 1
R1(config-router)# area 1 range
  *Area number is the area you are summarising FROM
 *#sh ip route - will show a NULL0 route to help prevent routing loops
 * #sh ip ospf database - will show the summary route

For Inter-area route summarisation, on the ASBR do:
R4(config)#router ospf 1
R4(config-router)#summary address
 *Advertises a summary route for all routes within subnet

Note - By default ABR's and ASBR's do not summarise routes.

Summarisation to be done on a Level1/2 router, which injects level 1 routes in to level 2.
If summarisation is being implemented, ALL level1/2 routers in the area need to be configured for summarisation.
Otherwise if one router advertises a more specific route then the Level 2 router will direct all traffic to that one router (due to the Longest Match rule).

On the Level1/2 router do:
R5(config)#router isis
R5(config-router)#summary-address level-2
 *You state which level router you are summarising in to (default is level-2).

BGP will not advertise a summary route unless the route can be first located in the Routing Table. As a result you can't simply apply a summary in the BGP process as it won't exist in the RT and therefore will not be advertised.

To get round this you first add a static route for the summary point to NULL0 and then you apply your summary route in BGP.

To do this do:
R2(config)#ip route NULL0
R2(config)#router bgp 100
R2(config-router)#network mask

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