The number of IPv4 prefixes in the global BGP table is approaching the limit of many Cisco products, such as 7600/6500 RSP720/Sup720 and some ASR1000, which may hold a maximum of ~500K routes in their FIB (the Forwarding Information Base, where only best paths are stored).
These routers can usually handle a bigger load of prefixes, they can also be used to receive the full BGP table from many upstream providers concurrently, but they can’t manage more than 500/512K entries in their RIB or FIB.
On 6500/7600 platforms, the default PFC3BXL/PFC3CXL TCAM configuration provides 512K entries for IPv4 and 256K for IPv6; the show mls cef maximum-routes command allows to show the running TCAM layout, that can be changed by entering mls cef maximum-routes ip … (it requires a reload).
7600(config)#mls cef maximum-routes ip 768 Maximum routes set to 786432. Configuration will be effective on reboot. 7600#show mls cef maximum-routes FIB TCAM maximum routes : ======================= Current :- ------- IPv4 + MPLS - 512k (default) IPv6 + IP Multicast - 256k (default) User configured :- --------------- IPv4 - 768k MPLS - 16k (default) IPv6 + IP Multicast - 120k (default) Upon reboot :- ----------- IPv4 + MPLS - 512k (default) IPv6 + IP Multicast - 256k (default)
While this method allows to patch the immediate requirement for more IPv4 FIB space, it has the side effect of reducing the IPv6 resources, which are supposed to be more and more needed in the future.
Some ASR1000 platforms support up to 500K IPv4 routes and, as far as I know, they don’t allow RIB/FIB repartition.
In some scenarios, in order to continue using these routers while avoiding performance impairments, the BGP – Selective Route Download feature may help.
The BGP—Selective Route Download feature allows a network administrator to selectively download some or none of the BGP routes into the Routing Information Base (RIB). (Cisco.com)
Suppose to have a border router which receives 3 BGP feeds: some prefixes from a multihomed customer, some from an Internet exchange and the full table from a transit provider. This border router is also connected to a core router, where other border routers and the rest of the network is connected too. Configuring the table-map … filter option on the border router allows to prevent its FIB to be loaded with a lot of useless prefixes, which may be summarized with a default route toward the transit provider.
Filtering may be based on many parameters; the one I used here is the next-hop address of each learned prefix. The final configuration looks like this:
router bgp xxx address-family ipv4 table-map BGP-Table-IPv4 filter ! route-map BGP-Table-IPv4 permit 10 match ip next-hop BGP-NextHop-OK route-map BGP-Table-IPv4 deny 10000 ! ip access-list standard BGP-NextHop-OK deny Transit_Provider_IP permit any
In order to minimize the changes to the standard configuration, an access-list is created which only denies the transit provider’s next-hop address.
Preferred prefixes learned from the Internet exchange and from the multihomed customer are permitted (not denied) by the access-list and, consequently, by the BGP-Table-IPv4 route-map referenced in the table-map command. Other prefixes are denied by the route-map and prevented from entering into the FIB, but they are anyway kept in the BGP database and announced toward the core router. Local-pref and other features may be configured on the routers to implement usual routing policies. Of course, in this example the core router must have a big enough FIB to handle the full BGP!
Please note that before configuring this feature, specific considerations are needed about local routing policies and traffic engineering mechanisms; special attention should be paid to locally generated prefixes announced to the border router by the rest of the network. As always, do it at your risk!
Cisco.com: BGP – Selective Route Download
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