| Junos Router | Global Route Preference | | Print | |
Routing information can be learned from multiple sources. In order to break ties among equally specific routes learned through multiple sources, each source is assigned a global preference. It can be said that the global preference determines the overall believability or "goodness" of a routing source. As such, routes that are learned through local administrative action—for example, static routes—are more believable than the same routes learned through a routing protocol such as OSPF. In Cisco IOS, this concept is called administrative distance.
| Source | Purpose | Default preference |
|---|---|---|
| Local | Local IP address of the interface | 0 |
| Directly connected network | Subnet corresponding to the directly connected interface | 0 |
| Static | Static routes | 5 |
| RSVP | Routes learned from the Resource Reservation Protocol used in Multiprotocol Label Switching (MPLS) | 7 |
| LDP | Routes learned from the Label Distribution Protocol used in MPLS | 9 |
| OSPF internal route | OSPF internal routes such as interfaces that are running OSPF | 10 |
| IS-IS Level 1 internal route | Intermediate System-to-Intermediate System Level 1 internal routes such as interfaces that are running ISIS | 15 |
| IS-IS Level 2 internal route | Intermediate System-to-Intermediate System Level 2 internal routes such as interfaces that are running ISIS | 18 |
| Redirects | Routes from ICMP redirect | 30 |
| Kernel | Routes learned via route socket from kernel | 40 |
| SNMP | Routes installed by Network Management System through the Simple Network Management Protocol | 50 |
| Router discovery | Routes installed by ICMP Router Discovery | 55 |
| RIP | Routes from Routing Information Protocol (IPv4) | 100 |
| RIPng | Routes from Routing Information Protocol (IPv6) | 100 |
| PIM | Routes from Protocol Independent Multicast | 105 |
| DVMRP | Routes from Distance Vector Multicast | 110 |
| Aggregate | Aggregate and generated routes | 130 |
| OSPF AS external routes | Routes from Open Shortest Path First that have been redistributed into OSPF | 150 |
| IS-IS Level 1 external route | Routes from Intermediate System-to-Intermediate System Level 1 that have been redistributed into ISIS | 160 |
| IS-IS Level 2 external route | Routes from Intermediate System-to-Intermediate System Level 2 that have been redistributed into ISIS | 165 |
| BGP | Routes from Border Gateway Protocol | 170 |
As with a route metric, numerically lower preference values are preferred. You can alter the default preference values when needed to accommodate some specific goal, such as route redistribution during an Interior Gateway Protocol (IGP) migration.
Readers familiar with Cisco Systems may note a few differences between how the two vendors assign distance/preference. For example, Cisco has a separate distance for Internal BGP (IBGP) versus External BGP (EBGP) (200 versus 20), whereas Juniper uses the same value. In this case, there is no operational impact because in the route selection process JUNOS software prefers EBGP over IBGP, resulting in the same behavior for both vendors. One area where the vendors differ is in regard to IGP versus EBGP distance. Here, Cisco assigns an OSPF IGP distance of 110; since this is higher than the EBGP distance of 20, it results in the selection of an EBGP route over an equivalent OSPF route. In the same setup, a Juniper router chooses the OSPF route.
Although you could alter JUNOS software preference to mimic IOS behavior, Juniper created a compatibility knob for this situation, called advertise-inactive. When applied to an EBGP peering session, this knob results in the advertisement of the best BGP route that happens to be inactive because of IGP preference. When using the advertise-inactive option, the JUNOS device continues to use the OSPF copy for forwarding, and the IOS device uses the EBGP copy to forward. However, from the perspective of an EBGP peer in a neighboring AS, both vendors appear to behave the same.
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