When CE-PE protocol is ISIS, we use “up/down” bit for loop prevention. This bit is the higher order bit in the default TLV 128 and 130. When a L1L2 router advertise a Level-2 prefix in the level-1 LSP, up/down bit is set.
When CE-PE protocol is non-link state protocol like EIGRP/BGP, Site of Origin (SoO) is used to uniquely identity a customer prefix and to avoid loop.
- When a PE router(say PE1) re-advertise an OSPF prefix from MP-BGP to CE router(say CE1) and if it Lype-3, Down bit “DN” in options field is set. Though RFC, says DN bit might be set for type-3,5, or 7 LSA, only for type-3 LSA, DN bit is set.
- This Down bit is preserved when propagating within customer site. If this prefix is advertised by a CE router to another PE router(say PE2), on seeing the down bit set on type-3,5 or 7 LSAs, it will not include the prefix in SPF calculation. But still resides in OSPF database of the VRF and aging mechanism takes place.
- Hence, PE2 has the prefix as learned via iBGP and not via OSPF, though OSPF(110) AD is lesser than iBGP(200)
when this summary LSA is advertised to PE2, following message would appear in “debug ip ospf spf”
- As the “down bit” is set only for Type-3 LSAs, we use “domain tag” to prevent routing loop for external LSAs.
- when a PE router redistribute an external Type-5/7 LSAs into OSPF, it adds an “external route tag” field.
: OSPF: Ignoring Type 5 External LSA 126.96.36.199, mask 255.255.255.255, adv 188.8.131.52 Tag equals to VPN Tag
- we can manually set this tag using “domain tag” CLI under OSPF configuration mode on PE router.
R6#show ip ospf database externalR6#show ip ospf database external
OSPF Router with ID (184.108.40.206) (Process ID 1)
Type-5 AS External Link States
Routing Bit Set on this LSA LS age: 131 Options: (No TOS-capability, DC) LS Type: AS External Link Link State ID: 220.127.116.11 (External Network Number ) Advertising Router: 18.104.22.168 LS Seq Number: 80000001 Checksum: 0xF73 Length: 36 Network Mask: /32 Metric Type: 2 (Larger than any link state path) TOS: 0 Metric: 20 Forward Address: 0.0.0.0 External Route Tag: 1200 HTH, venkat